236 EIsTTO-PA-RASITES AND HYGIENE. 



vouring the intermediate. The parasites which have re- 

 mained usually in the muscles of the latter, enclosed in a 

 cyst or capsule, are freed by the digestive liquids of the 

 stomach and begin at once their migration to their final 

 place of abode, where they then grow up to their full size. 

 Thus the life of these animals consists of two phases, rep- 

 resented by two different animals, one serving as habita- 

 tion for the immature form, the other for the adult in- 

 dividual; and it will not surprise us to find that the inter- 

 mediate host nearly always belongs to those animals 

 which serve as food for the final host. Thus the larva 

 of echinorhynchus lives in the wornil, which is the favor- 

 ite food of the pig. Indeed, some parasites can find 

 favorable conditions for further development in certain 

 animals only: they would perish if they happened to get 

 somewhere else — for instance, taenia saginata, the human 

 tape-worm, living in cattle, the tape-worm of the cat, in 

 the mouse, etc. At all events we see that these facts are 

 explained by the necessity of procuring or rather war- 

 ranting for the parasitic germs as much as possible the 

 ability of being transported into their final host in time 

 and successfully. 



But this is not all yet. In many cases this develop- 

 ment does not remain so simple; it is complicated by a 

 very strange phenomenon which again increases the im- 

 mense number of the germs and thus prevents anew a 

 possible extinction of the species. This is the "alterna- 

 tion of generations," which means that all those ani- 

 mals which are subject to it appear in alternating and 

 more or less different forms; the children do not resemble 

 the parents, but the offspring of these children resemble 

 Hgain their grandparents. Only one of the two forms is 

 differentiated sexually, being either separated into male 

 and female individuals or hermaphroditic; the second 

 form does not possess any developed organs of sexuality 

 ;ind produces its progeny always without sexual inter- 



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